While the answer from @manojlds was reasonable, I wasn't satisfied. The key question is not whether the seals on a microwave can be tested using Wifi at the same operating frequency (2.4GHz), but whether a cell phone is an accurate way of testing these same seals. In other words, will something that filters out 2.4GHz also filter out 900-1800MHz?
I did my own digging and am still not entirely satisfied, but wanted to report the results. I found a tutorial from Clemson University Vehicular Electronics Laboratory entitled "Practical Electromagnetic shielding", and tried to understand it. The key formula I thought might be helpful was this one:
Where where f is the frequency of the field and fc is the cut-off frequency of the opening. The cut-off frequency is approximately the frequency at which the maximum height or width, a, is equal to a half-wavelength. d is the depth of the opening.
I took this to mean that if all other characteristics about the shield are kept constant, decreasing the frequency will increase the attenuation -- perhaps an effective shield for 2.45 GHz really isn't an effective shield for lower frequencies.
I emailed the contact for the site, Dr. Todd Hubing, Michelin Chair and Professor of Electrical and Computer Engineering, and got the following response:
I would be a little concerned if I was able to communicate with a cell phone in my microwave oven. Nevertheless, it could happen I suppose. Traps and seals that work well at 2.45 GHz might not work so well at lower frequencies. It is true that apertures below cutoff have a shielding effectiveness that improves as frequencies get lower. However, the absorption loss associated with shielding materials gets worse at lower frequencies. Since microwave ovens operate in a very narrow frequency band, their shielding can also be narrow band.
So... not quite the definitive answer I hoped for, but reasonable.
I also posted in a forum called Microwaves101 under the Measurement and Testing sub-group. In response to my post, I got the following:
The door, specifically the surface around the cavity perimeter that it fits against, is tuned to 2.45 GHz. This surface is a 1/4 wavelength long (3 cm), so it acts as a choke specifically at that frequency. See section d of the below drawing.
See this wikipedia article on waveguide flanges:
I put my iPhone in the microwave and ran the WiFi sniffer app. WiFi signal (also at 2.45 GHz) was -60 dBm with the door open and -96 dBm with the door closed. 3G bars (4/5) did not change at all.
Anyway, that's 36 dB of attenuation (a 1/4000 factor) Assuming a 700W microwave, the power leaking out is 175 mW. A 19"x12" door is 1470 cm^2, so roughly, the power leaking out is 0.1 mW/cm^2. Health limit is 1 mW/cm^2, so the leakage is 1/10th of that. This is a really rough experiment, but you can see the leakage is safe.
So, there's where I've gotten. The best I can say is the following:
- An expert seems to think that it's possible that a) the microwave seals are targeted only toward the microwave operating frequencies, and b) tha the attenuation formula indicates that seals would perform worse on lower frequencies (in other words, theory supports the idea that a cell phone might ring while indicating nothing about the efficacy of the seals)
- An active microwave forum member conducted an experiment showing no loss in GSM bars and a reduction in microwave frequency with the door closed (practical experiment supports the theory, at least in one instance)